Engineers typically design high-pressure oil field pumps in two sections; the (proximal) power section (herein “power end”) and the (distal) fluid section (herein “fluid end”). The power end usually comprises a crankshaft, reduction gears, bearings, connecting rods, crossheads, crosshead extension rods, etc. In mud pumps, the power end also contains a liner within which a piston is moved in a reciprocating manner by a piston rod. Notwithstanding their location reversibly secured in the power end frame, liners (and the pistons and piston rods within them) are considered part of a pump's fluid end.
Commonly used mud pump fluid ends typically comprise a pump housing in which a suction valve and a discharge valve are associated with each liner (with its piston and piston rod) in a sub-assembly that also includes retainers, high-pressure seals, etc. A typical configuration includes three such subassemblies combined in a single pump housing. FIG. 1 shows a cross-sectional schematic view of a conventional mud pump liner having two component parts: an outer hull and an inner sleeve. See FIGS. 2A and 2B. The cylindrical inner surface of each sleeve seals against the peripheral surface of a piston seal to enable high pressure pumping.
Conventional mud pump liners were initially manufactured in a one-piece configuration from cast iron, a traditional wear-resistant bearing material. Cast iron liners were subject to corrosion and experienced rapid wear at pressures greater than about 1,000 pounds per square inch (psi), and they were replaced about 1950 by induction-hardened steel liners that had greater strength and wear-resistance, but the hardened steel had lower corrosion resistance compared to cast iron.
Chrome plating was then applied to steel liners to improve both corrosion resistance and wear resistance, and operating pressures increased to the range of 2,000 to 3,000 psi. Unfortunately the relatively thin chrome plating tended to crack at higher pressures, leading to rapid degradation and failure of the plating. Attempts to harden the underlying steel (as by carburizing) significantly raised manufacturing costs because warping induced during carburization required post-process grinding and honing that removed much of the carburized wear case.
In the 1980's, attempts were made to improve the service life of steel liners by ion nitriding the wearing surface. But the service life of such liners (typically comprising Nitriloy) was not materially improved. While the nitrided steel wear case had a hard surface (about 70 Rockwell C), it was both susceptible to corrosion and relatively thin. Early failure of the wear case exposed the softer steel underneath to rapid wear and, occasionally, catastrophic failure of the liner
Such catastrophic failures are almost unknown today, thanks to the wide use of industry standard liners comprising two parts: a chrome iron sleeve shrunk fit (or otherwise interference fit) within an outer steel hull. Chrome-iron sleeve liners offer several advantages over other types of liners. First, a relatively high level of free chrome in the sleeve assures good corrosion resistance and longer life. Second, relatively high carbon and chrome levels in the sleeve allow the formation of very wear-resistant chrome carbides. And since the sleeve has the same uniform hardness throughout its cross section, wear resistance does not decrease as the sleeve wears. Thus, catastrophic wear-through failures are almost entirely avoided, but these chrome-iron sleeve liners are relatively expensive and labor intensive to manufacture.
Even more expensive, but with a 300-400% increase in wear life over hardened steel liners, are ceramic and zirconium sleeve liners. Both ceramic and zirconium sleeve liners offer excellent corrosion resistance and uniformity of wear resistance throughout liner service life, as seen in chrome-iron sleeve liners. But ceramic and zirconium sleeve liners are very brittle, requiring delicate handling on a drilling rig where the work environment is far from delicate. Additionally, ceramic and zirconium sleeves have the disadvantage of being heat insulators. That is, they tend to store the substantial frictional heat that develops primarily due to movement of the piston's elastomeric seal material on the sleeve inner wall. While metallic sleeves tend to conduct at least a portion of this frictional heat away from the piston-sleeve interface, ceramic and zirconium liners tend to store the heat instead. Stored heat results in increased piston operating temperatures that degrade piston seals, eventually allowing a piston flange to contact the liner wall and damage it. Thus, chrome-iron sleeved liners remain the most popular choice for oil and gas field operations.
The chrome-iron used in industry standard liner sleeves typically comprises 25-28% chrome, 2.5% carbon, some trace elements, with the balance being iron. In some industries this alloy is referred to as “white iron.” The alloy has excellent wear and corrosion resistance, but chrome-iron sleeve liners are expensive and labor intensive to manufacture. The chrome-iron sleeve must be centrifugally cast, and because of the centrifugal force generated during casting, the favorable, heavier, alloy particles are primarily distributed closer to the outer diameter (OD) of the casting. Slag and other undesirable particles, on the other hand, are distributed on the inner diameter (ID) of the casting. Because the wear surface is on the ID, this particle arrangement after casting is just the opposite of the desired distribution. Thus the casting is made overly thick so the undesirable materials can be removed by machining that increases the ID.
But when the casting is removed from the centrifugal mold, the casting is at full hardness, approximately 60 Rockwell C, and can not be machined. Rather, the casting must first be annealed to a machinable state, which usually takes 24 hours in an annealing furnace. The casting is then rough machined, about one half the wall thickness being machined away to remove the undesirable particles from the casting ID. The casting is also cut into lengths at this time to make sleeves for the many different liner designs. Sleeves are then heat treated to regain the hardness of 60 Rockwell C, but since the sleeves warp during heat treatment, they must be returned to a near round condition.
Because of the hardness of the heat-treated (and out-of-round) sleeves, they cannot be machined. Instead, they must be ground on their OD. After grinding, the sleeve OD is measured and a steel hull is bored to an ID dimension slightly smaller than the OD of the ground sleeve. The hull is then heated to approximately 500-700° F.; at which temperature the hull ID increases so that it exceeds the OD of the ground sleeves. The ground sleeve is slipped into the ID of the hull, and as the sleeve-hull (i.e., the liner) assembly cools the hull shrinks around the sleeve to lock it in place and place the sleeve in compression via the hoop tension developed in the hull as it shrinks. After cooling, the sleeve ID is honed to bring its ID to one of several standard sizes within American Petroleum Institute (API) size tolerances. The hull OD is then machined to the final design dimensions for liners used in a particular pump.
Liners made according to the above process are much more durable than the original one-piece cast iron liners, but also much more expensive. An improved liner is needed that will substantially equal or outperform the current industry standard liner while reducing manufacturing cost.